Liquid crystal displays have been broadly used as image displays applied to portable telephones, personal computes or the like. A known LCD (liquid crystal display) is equipped with a surface light source device for backlighting a LCD panel thereof. Usually, a surface light source device is provided with a light guide plate for taking in light from a light source and then direction-converting the light as to be outputted. Light traveling within the light guide plate after being taken therein involves a gradual emission from an emission face.
Surface light source devices of this type are disclosed in Tokkai 2001-108835 (Document 1) and Tokkai-Hei 11-119219 (Document 2).
A surface light source device shown in Document 1 has a light guide plate having a back face (a face opposite to an emission face) provided with first prismatic projections, arranged repeatedly, running parallel to an incidence face (incidence side end face) of the light guide plate. This intends to give a uniformalized brightness to the emission from the emission face of the light guide plate by contriving angles of reflection surfaces provide by the projections or repetition pitch of them.
In addition the emission face of the light guide plate of the surface light source device shown in Document 1 is provided with second prismatic projections, arranged repeatedly, running perpendicularly to the incidence face (incidence side end face) of the light guide plate. The second prismatic projections helps the emission to be gathered and have an increased brightness.
A surface light source device shown in Document 2 has a light guide plate which contains a light diffusible material dispersed therein and has a back face (a face opposite to an emission face) provided with a plurality of prismatic projections. These prismatic projections correct directivity of illumination light emitted from the emission face. In addition, the emission face is provided with light scattering pattern for promoting light-outputting and uniformalizing emission brightness.
However, the surface light source device disclosed in Document 1 hardly able to realize an efficiently high emission brightness (intensity) while emission brightness is uniformalized.
In the case of the surface light source device disclosed in Document 2 above, emission directivity corrected by the prismatic projections formed on the back face of the light guide plate is disturbed on light-outputting by light scattering caused on light-outputting by the light scattering pattern formed on the emission face of the light guide plate, although an effectively uniformalized brightness is obtained. As a result, a required emission intensity is hardly obtained.
In general, a light guide plate has a back face inclined so that plate thickness is decreasing from an incidence side end face to an opposite end side face, providing a wedge-like cross section. Therefore, light traveling within the light guide plate has an inner-incidence angle getting smaller with an increasing distance from the incidence side end face. This gives the light guide plate characteristics such that light-outputting from the emission face tends to get easier with an increasing distance from the incidence side end face. Accordingly, it has been desired to improve structure of light guide plate so that high and uniform brightness is realized under consideration of the above characteristics.